1. Field of the Invention
The present invention relates to a scintillator single crystal and a production method thereof, and more particularly, to a scintillator single crystal used in a single crystal scintillation detector (scintillator) for detecting radiation such as gamma radiation and the like in fields such as radiology, physics, physiology, chemistry, mineralogy, and oil exploration, including medical diagnostic positron CT (PET) scanning, observation of cosmic rays, and exploration for underground resources, and to a production method of such a scintillator single crystal.
2. Related Background Art
Because scintillators that use cerium-doped gadolinium orthosilicate compounds have a short fluorescent decay time and also a large radiation absorption coefficient, they have found applications as radiation detectors for positron CT and the like. The light output of these scintillators is greater than that of BGO scintillators, but only about 20% of the light output of NaI (Tl) scintillators, and further improvement is needed in that area.
Recently, scintillators using single crystals of cerium-doped lutetium orthosilicate, which are generally represented by the formula LU2(1−x)Ce2xSiO5, have been disclosed (Japanese Patent Registration No. 2852944, and U.S. Pat. No. 4,958,080). In addition, scintillators using single crystals of the compound represented by the general formula Gd2−(x+y)LnxCeySiO5 (wherein Ln represents Lu or a species of rare earth element) have been disclosed (see Japanese Patent Application Laid-open No. 7-78215 and U.S. Pat. No. 5,264,154). It is known that in these scintillators not only is the crystal density increased, but also the light output of cerium-doped orthosilicate single compounds crystals is increased, and the fluorescent decay time can be shortened.
However, in the case of growing or cooling specific cerium-doped silicate single crystals in an atmosphere containing oxygen (e.g., atmosphere having an oxygen concentration of 0.2 vol % or more), or in the case of growing in atmosphere having a low oxygen concentration, and the single crystals are subsequently subjected to high-temperature heat treatment in an atmosphere containing oxygen, this has been clearly shown to lead to a decrease in light output due to coloring of the crystals, absorption of fluorescence and so forth (see Japanese Patent Registration No. 2701577). In addition, when growing cerium-doped silicate single crystals, the Czochralski method is typically used that employs high-frequency heating using an Ir crucible due to the high melting point of the single crystals. However, since Ir crucibles ends up evaporating when heated to a high temperature in an atmosphere containing oxygen, there is the problem of difficulty in achieving stable crystal growth.
Japanese Patent Registration No. 2701577 discloses a heat treatment method that increases scintillator properties such as light output and energy resolution capability and the like of single crystals of cerium-doped gadolinium orthosilicate compounds. This heat treatment method is one wherein a heat treatment is performed in an oxygen-poor atmosphere at a high temperature (a temperature ranging from 50° C. to 550° C. lower than the melting point of the single crystal). According to this document, scintillation properties are increased by an effect wherein tetravalent cerium ions, which are an impediment to scintillation luminescence, are reduced to a trivalent state.
Japanese Patent Application Laid-Open No. 2001-524163 discloses silicate crystal-based scintillation materials containing Lu and Ce, which include oxygen vacancies □ and are represented by the following general formula (8):Lu1−yMeyA1−xCexSiO5−z□z  (8)(wherein, A represents at least one element selected from the group consisting of Lu, Gd, Sc, Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm and Yb, Me represents at least one element selected from the group consisting of H Li, Be, B, C, N, Na, Mg, Al, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, U and Th.)This document describes fifty and more elements (ions) from H to Th as Me substitute for Lu. The document describes that introduction of these elements prevents cracking of crystals during cutting and manufacturing scintillation elements and creates waveguide properties in waveguide elements. The document also describes that existence in a scintillation material or addition into the material of a necessary quantity of ions having the oxidation of +4, +5 and +6 (for example, Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, W, W) allows to prevent cracking of crystals and forming of vacancies in oxygen sub-lattices.
Japanese Patent Publication No. S64-6160 discloses that a method for heating single crystals of tungsten acid compound in an oxgen-containing atmosphere in the range of temperature as follows;(Tmp−200)≦T<Tmp(wherein T is heating temperature, Tmp is the melting point of the crystal)as a heating treatment for increasing light output of an oxide scintillator. This document describes that although the crystal is easy to create oxygen vacancies, the light output of the tungsten single crystal increases by heating the crystal near the melting point in the oxygen-containing atmosphere to decrease oxygen vacancies in the crystal.
Japanese Patent Application Laid-Open No. 2003-300795 discloses a single crystal of Gd(2−x)CexMeySiO5 (wherein x is 0.003 to 0.05, y is 0.00005 to 0.005, Me is an element selected from a group consisting of Mg, Ta and Zr or their mixture). The document discloses that the elements represented by Me prevent Ce ions from changing their valencies from 3+ to 4+ to obtain an achromatic and highly transparent single crystal.